1. Field of the Invention
The present invention relates to a surface acoustic wave element including a laminated substrate, a method for producing the same and a surface acoustic wave device using the same.
2. Description of the Related Art
In the development of mobile communication equipment, it is desired to achieve high performance of a surface acoustic wave element, which is one of the key devices constituting the equipment. In the case where a band in which signals are sent out is close to a band in which signals are received, as seen in the recent mobile communication systems, it is difficult to achieve a sharp cut-off property in the temperature range used. This is due to the characteristics of a piezoelectric substrate used in conventional surface acoustic wave elements. Specifically, this is because although conventional piezoelectric substrates have a coupling coefficient sufficient to achieve a required band width of the system, in general they have a large frequency temperature coefficient. In order to cope with this problem, it was reported that attaching an existing piezoelectric substrate to an auxiliary substrate having a thermal expansion coefficient different from that of the piezoelectric substrate provides a surface acoustic wave element having a large coupling coefficient and excellent temperature stability (Proc. 1997 IEEE Ultrasonics Symposium, pp. 227-230).
Hereinafter, a conventional surface acoustic wave element will be described. FIG. 13A is a perspective view of an example of a conventional surface acoustic wave element, and FIG. 13B is a cross-sectional view thereof taken along line Zxe2x80x94Z in FIG. 13A. Referring to FIGS. 13A and 13B, the conventional surface acoustic wave element includes a first substrate 401, a second substrate 402, a comb-shaped electrode 403 including electrodes 403a and 403b, and a reflector 404. As the first substrate 401, a 36xc2x0 Y-cut X-propagating lithium tantalate single crystal is used, for example. As the second substrate 402, a glass substrate having a thermal expansion coefficient smaller than that of the first substrate 401 in the propagation direction of a surface acoustic wave is used, for example. The thickness of the first substrate 401 is sufficiently smaller than that of the second substrate 402, and is sufficiently larger than the wavelength of the surface acoustic wave. For example, the first substrate 401 is about 40 xcexcm thick and the second substrate 402 is about 310 xcexcm thick. The first substrate 401 and the second substrate 402 are joined together substantially directly without an adhesive or the like therebetween. Such a structure allows control of the frequency temperature coefficient while maintaining the characteristics of the existing piezoelectric substrate.
However, the conventional surface acoustic wave element as described above has the following problems. Since it has a laminate structure where the piezoelectric single crystal several tens of xcexcm thick and the glass substrate are laminated, it is difficult to handle the surface acoustic wave element. For example, in mounting the surface acoustic wave element on a package, in particular, in picking up the surface acoustic wave element, cracking or fracturing may occur in the piezoelectric single crystal layer. Moreover, in dividing a wafer into individual surface acoustic wave elements, when it is cut with a cutting blade selected based on the glass substrate, fracturing or chipping may occur in the piezoelectric single crystal portion during cutting because of the difference in the characteristics between the materials.
It is an object of the present invention to provide a surface acoustic wave element that can be handled easily and has few defects and a surface acoustic wave device using the same. It is another object of the present invention to provide a method for producing a surface acoustic wave element that allows a surface acoustic wave element to be produced reliably and inexpensively by preventing the generation of defects in the process of separating into elements.
In order to achieve the above objects, a surface acoustic wave element of the present invention includes a laminated substrate where a first substrate made of a piezoelectric material is laminated over a second substrate made of a material different from that of the first substrate, and includes at least one pair of comb-shaped electrodes formed on one main plane of the first substrate. A step or a notch is formed on the periphery of the laminated substrate on the side of the first substrate. This embodiment provides a surface acoustic wave element that can be handled easily and has few defects.
In the above-surface acoustic wave element, it is preferable that the first substrate is made of a piezoelectric single crystal. This embodiment provides a surface acoustic wave element having a small propagation loss.
In the above-surface acoustic wave element, it is preferable that the second substrate is made of glass. This embodiment provides the laminated substrate with a high joining strength, and thermal stress can be applied effectively onto the surface of the first substrate. Therefore a surface acoustic wave element having excellent temperature stability can be obtained.
In the above-surface acoustic wave element, the first substrate may be laminated directly on the second substrate.
In the above-surface acoustic wave element, the first substrate may be laminated over the second substrate via an adhesive. This embodiment increases the degree of freedom in selection of the type of the substrate and provides a surface acoustic wave element that can be produced particularly easily.
In the above-surface acoustic wave element, the step or the notch may be formed only in the first substrate.
In the above-surface acoustic wave element, the step or the notch may be formed from the first substrate into the second substrate.
Furthermore, a method for producing a surface acoustic wave element of the present invention includes (a) forming a laminated substrate where a first substrate made of a piezoelectric material and provided with a pair of comb-link electrodes on one main plane thereof is laminated over a second substrate made of a material different from that of the first substrate; (b) forming a groove on a portion around the comb-shaped electrodes of the laminated substrate; and (c) grinding at an approximately central portion of the groove on the laminated substrate in a width narrower than that of the groove so as to cut the laminated substrate. This embodiment allows the surface acoustic wave element of the present invention to be produced reliably at a low cost.
In the above method, it is preferable that in the process (b), the groove is formed by grinding the laminated substrate, and the grinding in the process (b) is performed at a speed lower than that of the grinding in the process (c). This embodiment suppresses the generation of defects in the first substrate made of a piezoelectric material, and allows the surface acoustic wave element to be produced in high productivity.
In the above method, the groove may be approximately V-shaped in section. According to this embodiment, the notch can be formed on the periphery on the side of the first substrate.
Furthermore, a surface acoustic wave device of the present invention includes a substrate and a surface acoustic wave element mounted on the substrate, wherein the surface acoustic wave element is the surface acoustic wave element of the present invention.
In the above-surface acoustic wave device, it is preferable that the surface acoustic wave element is mounted on the substrate in a face-down manner. This embodiment provides a surface acoustic wave device without degradation of the characteristics.
As described above, in the surface acoustic wave element of the present invention, the step or the notch is formed on the periphery on the side of the first substrate made of a piezoelectric material. Therefore, the present invention provides a surface acoustic wave element that can be handled easily and has few defects.
Furthermore, the method for producing a surface acoustic wave element of the present invention allows the surface acoustic wave element of the present invention that can be handled easily to be produced reliably and at a low cost.
Furthermore, the surface acoustic wave device of the present invention can be produced easily and has high reliability, because the surface acoustic wave element of the present invention is used.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.